1. Field of the Invention
The present invention relates to an image sensor unit, and more particularly, to an improved structure of a continuously dischargeable light source used in the image sensor unit, which can improve the starting characteristic of the light source for achieving stable glow discharge (that is, so-called continuous electric discharge).
2. Description of the Related Art
In general, photocopy machines, facsimile machines, scanners, and other image-reproducing machines use an image sensor unit to optically read the image from the original. Some image sensor units use a certain type of light source that makes use of continuous electric discharge occurring between electrodes, instead of utilizing the light emitting phenomenon of thermal particles from a filament.
One example of the light source making use of the continuous electric discharge comprises a light-emitting body made of a dielectric material and shaped as an airtight container, filled with xenon gas, for example under a prescribed vacuum atmosphere. In the container, a fluorescent material layer is provided, except for a certain region, and two electrodes are arranged so as to face each other with a space between them. When a discharge starting voltage is applied between the electrodes, a strong electric field is generated between them, and the gas filled in the container is ionized and excited to produce ultraviolet rays. The ultraviolet rays cause the fluorescent material layer to emit light, functioning as the light source.
If the object to be optically scanned, such as an original, has a flat surface, a plate glass with a flat surface is used in the image sensor unit to hold the scanned surface of the original. In this case, the light emitting structure described above is placed below the plate glass so as to irradiate the original from the bottom.
It is important for the above-described type of image sensor unit to reliably provide the starting characteristic of the light source. Accordingly, it is proposed to create a localized area of the strong electric field to enhance the electric discharge efficiency, thereby securing continuous light emission across the entire range (see, for example, JPA 11-283579). It is also proposed to use an additional light source to irradiate the light emitting body in order to ionize and excite the gas filled in the light emitting body (see, for example, JPA 4-106896). Still another proposal is to mix a material that easily emits electrons in the fluorescent material used in the light emitting body, instead of irradiating the light emitting body (see, for example, JPA 2000-156203 and JPA 2001-123988). Yet another proposal is that a portion of the fluorescent layer be removed so as to allow initial electrons to emit from this removed portion for the purpose of starting ionization and excitation of the gas (see, for example, JPA 2001-102004). The emission of the initial electrons is compared to a “small flame” for sequentially causing ionization and excitation to achieve continuous electric discharge.
However, there are several problems in the above-described prior art techniques of enhancing the starting characteristic.
First, the conventional method of generating a strong electric field requires auxiliary electrodes to generate such a strong electric field, and the configuration of the light emitting body, including the ordinary electrodes, has to be changed. This results in increased cost. Particularly, if the height of the discharge space defined by the light emitting body is insufficient, the insulating distance of the auxiliary electrodes cannot be maintained reliably. This drawback may cause dielectric breakdown, and therefore, cause the light source to return to the dark state, causing dark current. In addition, the lighting circuit may be damaged, or the inside of the image sensor unit may be burned. Consequently, the starting characteristic cannot be improved efficiently.
Second, with the prior art technique using an additional light source, or mixing a material that easily emits electrons in the fluorescent layer, the emitted electrons are likely to be captured if the purity of the gas filled in the light emitting body is degraded. Since this phenomenon adversely affects the efficiency of the ionization and the excitation of the gas, the purity of the gas has to be maintained high. In addition, when a portion of the electrode is exposed, sputtering occurs on the exposed surface of the electrode, and the illumination of the light source decreases.
Third, with the prior art technique of removing a portion of the fluorescent material layer to expose the underlying layer or material to the gas, the quantity of light may become uneven or insufficient within the scanning range of the original, which is the scanning target of the image sensor unit. This may cause the S/N ratio to be degraded. In addition, since most of the surface of the light emitting body made of a dielectric is covered with the fluorescent material layer, the voltage applied between the electrodes has to be increased in order to guarantee initial emission of electrons. It is difficult for the light source of this technique to maintain electric discharge continuously along the longitudinal axis of the light emitting body, even if weak electric discharge has occurred at the exposed portion, and in the worst case, electric discharge may stop.
Therefore, it is an object of the present invention to overcome the above-described problems in the conventional image sensor units, especially in the light emitting part making use of continuous electric discharge, and to provide an image sensor unit with an improved starting characteristic that can prevent undesirable dielectric breakdown and cost increase. This image sensor unit has a light emitting body that is capable of providing continuous electric discharge across the entire scanning area of the light emitting body, while preventing the degradation of the S/N ratio.